The Ellipsoidal Reflector Spotlight (ERS) and the Parabolic Wash light (PAR) are two of the most popular lighting fixtures used in theatre, television, and architectural lighting. Certain ERS and PAR lighting fixtures employ an incandescent High Performance Lamp (HPL) that includes a plurality of linear, helically-wound filaments arranged with their longitudinal axes substantially parallel with each other and arranged with their longitudinal axes spaced substantially symmetrically about a central longitudinal axis. Such ERS and PAR lighting fixtures typically have a reflector to collect the light from the lamp and direct it forward out of the fixture.
When an HPL lamp is used with a reflector having a circularly symmetric cross section, the irradiance distribution of the forwardly directed beam is not circularly symmetric. Rather, the irradiance distribution is characterized by a number of “hot spots” surrounding a central minimum, or void. The number of hot spots is equal in number to the number of helically wound filaments contained in the HPL lamp. The central minimum is due to the fact that no light emitting element is located along the lamp's central longitudinal axis.
FIGS. 1A, 1B and 1C show an HPL lamp 100 with four helically-wound filaments 102, 104, 106 and 108 arranged with their longitudinal axes substantially parallel with each other and with their longitudinal axes spaced substantially symmetrically about a central longitudinal axis 105. The lamp includes a base 110, a transparent enclosure 112, and a pair of insulating filament support structures 116. FIG. 1C shows an electrical conductor 117 connecting the filaments so that they operate electrically in series.
The four emitting filament sections 102, 104, 106 and 108 are composed of tightly wound helical coils, each of which emits light and in certain directions, block the light emitted from other filaments, creating shadows. These shadows are visible in the lamp's intensity distribution. Intensity is defined as the lumens or power directed into a given solid angle. The filament shadows create inconsistencies in a beam of light formed by collecting the light from the lamp and directing it forward with a typical prior art reflector.
FIG. 2 illustrates an intensity distribution graph 200, measured in a plane that is perpendicular to the HPL lamp's central longitudinal axis. The intensity data in the graph are generated in the following manner. The HPL lamp is burned base downward in a vertical orientation, while an intensity meter is moved in a circle centered on the lamp's central longitudinal axis. At each angle around the circle, the intensity is noted. Plotting the intensity versus angular position on a set of polar axes produces the intensity distribution graph 200. A number of filament shadows 202, 204, 206, 208, 210, 212, 214 and 216 are present in the beam. For clarity, the positions of the HPL filaments 102, 104, 106 and 108 are superimposed on the graph.
Prior art Ellipsoidal Reflector Spotlight (ERS) optical systems that employ the HPL lamp also employ a reflector generated from an ellipsoidal or near-ellipsoidal curve, typically referred to as an ellipsoidal reflector. A generator curve is rotated about the lamp's central longitudinal axis to form a reflecting surface. FIG. 3 shows an ellipsoidal or near-ellipsoidal generator curve 302, an HPL lamp 100 and its central longitudinal axis 105, and an optical axis 304 of the ERS system, which is coincident with the lamp's central longitudinal axis 105. A dotted curve 308 depicts the opening of the reflecting surface, and a circular arrow 310 depicts the sense of rotation as the generator curve 302 is swept around the optical axis 304 of the ERS system.
FIG. 4 shows an orthogonal view of a reflecting surface 400 resulting from application of the prior art method described with regard to FIG. 3. The reflecting surface 400 has a smooth bowl-like shape. Since the generator curve 302 was rotated about the optical axis 304 of the ERS system, a cross section 402 taken through the reflecting surface 400, in a plane perpendicular to the optical axis 304 of the ERS system, is a circle.
As described with regard to FIG. 2, the light emitting filaments in the HPL lamp shadow each other. When a smooth reflector is employed in the ERS optical system, the filament shadows tend to be visible in the forward propagating beam projected by the reflector. For this reason, prior art ERS reflectors for use with HPL lamps often employ facets or lunes on their surface in an attempt to produce a circularly symmetric irradiance distribution, free from filament shadows and hot spots.
Facets are small planar segments, which are tiled over the reflector's surface. Lunes are ribbon-like segments, which have curvature in one direction only, and are also tiled over the reflector's surface. The facets or lunes are perturbations of the smooth reflecting surface profile, and therefore help to smooth or homogenize the beam formed by the lamp and reflector. FIG. 5A shows a reflector section 502 that has been covered with facets 504. FIG. 5B shows a reflector section 506 that has been covered with lunes 508.
A reflective surface formed according to the prior art method described with regard to FIG. 3 and faceted or luned according to the methods described with regard to FIG. 5 has a generally circular cross section 402. But the cross section 402 is actually a polygon whose number of sides is equal to the number of facets or lunes through the cross section of the reflective surface 400.
A small number of large facets or lunes tend to smooth the irradiance distribution in the projected beam by filling in the filament shadows present in the lamp's intensity distribution. However, large facets or lunes also cause a significant deviation from the reflector's original shape, resulting in a decrease in the efficiency of the optical system. The efficiency of an optical system is defined as the ratio of the power in the projected beam to the power of the lamp. To preserve efficiency, and minimize the deviation from the reflector's original shape, the size and number of facets or lunes are often varied across the reflector's surface. Such an arrangement smoothes the irradiance distribution of the projected beam, while minimizing the impact on the optical system efficiency. FIGS. 6A and 6B show orthogonal and front views, respectively, of a prior art ERS reflector 600 whose surface is covered with lunes of differing sizes and numbers in regions 602, 604 and 606.
FIG. 7 presents a schematic view of a prior art ERS projection optics system 700. The ERS optical system 700 includes an HPL lamp 100, a luned ellipsoidal reflector 600, a projection gate 702, and a projection lens 704. The projection lens 704 forms an image 706 of the projection gate, or any object placed therein, on a distant projection surface 708. Objects placed in the projection gate may be referred to as gobos. Since the projection lens 704 forms an image of the projection gate 702, the radiometric characteristics of the beam at the gate location are conveyed to the image 706 formed on the projection surface 708. Therefore, if the beam in the projection gate does not have a desired irradiance distribution, the image projected on the distant surface will not have the desired irradiance distribution.
FIG. 8 presents a contour map 800 of the irradiance distribution of the beam formed by the ERS optical system 700 on the distant projection surface 708. As described with regard to FIG. 7, this irradiance distribution is also descriptive of the irradiance distribution of the beam at the projection gate 702. Each contour 802 on the contour map 800 defines a zone of constant irradiance, or an isoirradiance contour. Although the ERS reflector surface is luned, the beam at the projection gate does not have a uniform irradiance distribution.
As the isoirradiance contours in FIG. 8 show, the irradiance distribution is not circularly symmetric. The irradiance distribution has a minimum, or void, at the center 804, which is due to the fact that no emitter is located on the central longitudinal axis of the HPL lamp. Furthermore, the irradiance distribution has four hot spots 806, 808, 810 and 812, which coincide with the location of the four emitting filaments in the HPL lamp.
Thus, the irradiance distribution map 800 is rotationally symmetric, that is, if rotated by some multiple of ninety degrees the resulting map is substantially identical to the original map. However, the map is not circularly symmetric. If rotated by an arbitrary number of degrees (specifically an angle other than a multiple of ninety degrees) the rotated map is not substantially identical to the original map.
A Parabolic Wash light (PAR) may also employ the HPL lamp. In a PAR optical system, a parabolic or near-parabolic curve is rotated about the longitudinal axis of the optical system to form a reflecting surface, typically referred to as a parabolic reflector. Because the generator curve is parabolic or near-parabolic, a beam exiting the reflector is substantially parallel to the optical axis of the PAR system. That is, the light beam is made up of light rays that are substantially parallel to each other and to the optical axis.
A PAR optical system typically consists solely of a reflector and lamp, although a lens may be placed after the reflector to further smooth or shape the beam. As described with regard to an ERS optical system, a parabolic reflector surface in a PAR optical system may be covered with facets or lunes in an attempt to project a beam with a smooth irradiance distribution. However, the irradiance distribution of a beam produced by such a PAR optical system has a central void and four hot spots, as described with regard to an ERS optical system.